The invention relates to a matrix switch having a piezoelectric material for converting key depressions or the like into electrical signals.
It is well known in the art that a thermoplastic film such as, for example, a pressed poly-y-methyl glutamate film, and a polarized polyvinyl fluoride or polyvinyliden fluoride film has a piezoelectric property. Keyboard switches in which these piezoelectric plastic films are used, have also been proposed, for example, in Japanese Opened Patent Application No. 47-37244 and in Japanese Patent Application No. 48-118801.
Since a piezoelectric thermoplastic film of a desired area is easily obtainable and has a relatively low Young's modulus and hence high elasticity, a keyboard switch having any desired number of stress action points can be produced easily and is applicable for use as a coded signal input device.
When a coded signal is read out from the keyboard by pushing one of the keys arranged on the keyboard, at least two kinds of signals, i.e. a row signal and a column signal, are obtained in accordance with the selected key, and the position of the key can be identified by the combination of the obtained signals. Such keyboard switch basically comprises a substrate and a piezoelectric film disposed on the substrate, wherein two groups of parallel conductors are placed on the upper and lower surfaces, respectively, so as to provide cross points at which the respective keys are positioned, e.g. as shown in FIG. 7. In such keyboard, the conductors to as X'1, X'2, X'3 . . . X'm are arranged in rows and the conductors Y'1, Y'2, Y'3 . . . Y'n, are arranged in columns. The cross points, for example, (X'1, Y'1) can be identified as the crossing point of the conductors X'1 and Y'1. When a field effect transister (FET) is to be driven by a piezoelectric field produced at, for example, X'1, it is usual to connect the conductor X'1 to a gate of the FET and to ground a corresponding electrode Y'1 on the backside surface of the film. Since the electrode Y'1 is used to drive another FET, and thus should not be grounded directly, a very complicated circuit means is required as the indentification circuit for the position (X'1, Y'1).
An alternative structure of a keyboard switch comprises a common ground electrode on the under surface of the piezoelectric film, and at least two simultaneously operating separated electrodes in the respective key positions. In the case where two electrodes are provided as shown in FIG. 8, it is usual that the electrodes arranged in each of rows X"1, X"2 . . . X"m are connected with one another, and that the electrodes arranged in each of columns Y"1, Y"2 . . . Y"n are also connected with one another. The row and column electrodes are respectively connected to the ground electrode. In this arrangement, the keys on a row or a column can be read out by means of a reading circuit. Therefore, only n + m reading circuits are required for a n .times. m matrix. This substantially reduces the number of the switch circuits required. On the other hand, the row electrodes and the column electrodes placed will intersect in a matrix of more than 2 .times. 2, and therefore cannot be placed on a common plane. In order to prevent contact between the row and column electrodes, it is necessary to connect each electrode of either the row electrodes or column electrodes to contacts on the under surface of a printed circuit board so as to place the row electrodes and the column electrodes on separated planes. Further, in the case where the row electrodes and the column electrodes are placed on a common plane and where two electrodes are placed in one key position, comb shaped electrodes may be used as shown in FIG. 8. The comb shaped electrodes can prevent the piezoelectric fields of different magnitudes from being generated in two key positions when the key is pushed with a slight shift.
A keyboard switch using a piezoelectric film is advantageous in that it can be manufactured simply by laminating the film provided with a plurality of key operating points on a base plate. However it is disadvantageous in that each of either the row or column electrodes in required to be connected to each operating point of a printed circuit board and in that electrodes having a complex structure, such as comb shaped electrodes, are required.